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Note: Watch the live webcast of the TSB's derailment report Tuesday at 10:30 a.m. ET by clicking here or tapping on the link under Related Stories.

MONTREAL - The Transportation Safety Board is expected to release its final report on the Lac-Mégantic train derailment on Tuesday.

The agency’s investigators arrived in Lac-Mégantic hours after a train with 72 tanker cars hauling crude oil derailed and exploded at the centre of town early in the morning of July 6.

During the year-long effort in Lac-Mégantic, the TSB assigned to the file roughly 50 investigators and experts in dangerous goods, engineering, human error, railway transport and mechanics.

After the 150 firefighters working on the scene brought the oil fire under control — which took about four days — search and rescue teams, TSB investigators and provincial police began looking through the wreckage for bodies and evidence that could helpdetermine the cause of the accident.

During the investigation, TSB employees interviewed witnesses and railway employees, inspected the train’s black boxes and analyzed oil from the Lac-Mégantic rail cars and from wells in North Dakota, where the oil originated.

They examined kilometres of track, took samples of the tank cars to determine their composition and made 3D laser scans of all 72 cars to figure out exactly how they failed.

Investigators focused on several factors they believed may have influenced or caused the accident: the railway cars and infrastructure, the oil, and possible human error.

The Montreal, Maine and Atlantic train travelling through Lac-Mégantic on July 6 was hauling crude oil bound for an Irving refinery in Saint John, N.B.

The TSB and its U.S. counterpart, the National Transportation Safety Board, have both noted in dozens of accident reports that the cars are prone to failure in accidents.

In a set of recommendations released in January, the TSB wrote that “for about 20 years” the agency had warned that the DOT-111 class cars are likely to release their contents in a crash.

“In previous investigations, the TSB identified the risks posed by the release of product from Class 111 tank cars, and the vulnerability of these cars to release product due to accident damage. The weaknesses of Class 111 tank cars have been acknowledged by the regulator and industry,” the TSB wrote.

Almost all of the cars involved in the crash were breached and released oil.

Four of the cars also suffered thermal tears — shell failures caused by extreme pressure inside the tanks that built up as the fire heated and vaporized the oil.

Two of these cars tore within the first 20 minutes of the fire, TSB investigators found. With flammable material, this type of tank failure can be catastrophic and cause “large fire balls,” the agency explained in a lab report released in March.

In their recommendations released in January, the agency suggested that the federal government require railways transporting flammable liquids in DOT-111 class cars to improve their cars to meet new, stricter safety standards introduced in 2011.

Those standards require thicker shells and improved fittings on the cars. None of the Lac-Mégantic cars were equipped with these features.

Investigators also tested the crude oil the 72 tankers carried.

Several firefighters on the scene of the accident last year observed that the oil burning in Lac-Mégantic seemed unusually flammable. Crude oil is not typically easy to ignite, and some guessed that the train had been carrying natural gas or another more reactive substance.

Transportation Safety Board investigators also said that they were surprised by how readily the oil had ignited.

Investigators sampled oil from nine of the train’s cars, which did not explode and were moved to Nantes, uphill from Lac-Mégantic, after the derailment. The oil contained in these cars, which were not damaged in the accident, provided a more accurate sample than oil that had been heated by the fire, the TSB said.

TSB analysis confirmed that the oil was more volatile than the train’s shipping manifest suggested.

The oil had been labelled as a packing group III substance — the least volatile classification for flammable liquids, which are categorized under Canada’s Transportation of Dangerous Goods regulations. But TSB analysis found that its properties would place it in the more volatile packing group II.

The oil’s flashpoint — the temperature at which vapours would ignite — was closer to gasoline, the TSB said.

The TSB recommended in January that Transport Canada require railways to do regular risk assessments and adopt stricter safety measures, including speed reductions and more regular inspections, for all dangerous cargo routes.

The agency also recommended that railways be required to write emergency response assistance plans for all trains carrying large amounts of oil, which was not a requirement in July 2013.

The TSB’s report is also expected to address human factors and infrastructure problems that might have led to the accident.

At the time of the accident, MMA was one of two railway companies in Canada using one-person crews.

The train that derailed in Lac-Mégantic was crewed by just one employee, engineer Thomas Harding.

MMA chairman Ed Burkhardt said after the accident that the company believed one-person crews were safer, because employees were less likely to be distracted.

Around 11 p.m. on July 5, Harding parked the train on the main track in Nantes before going to a motel to sleep for the night.

He later told police that he turned on the train’s air brakes and set seven hand brakes. According to police documents, the company’s guidelines required that at least nine brakes be set.

Another MMA employee told police in an interview that 10 to 15 brakes should have been applied.

After shutting down the train to test the hand brakes, Harding left one engine running to maintain power to the air brake system, and then took a taxi to a hotel in Lac-Mégantic.

According to Harding, the lead engine — which he left running — had been smoking throughout the day.

Parked in Nantes, the engine was releasing a cloud of oil so thick that the taxi driver who met Harding had to use his windshield wipers to remove oil from his car before driving back to Lac-Mégantic.

Several MMA employees interviewed by police said that before the accident they had been worried about the poor state of the company’s equipment and allegedly infrequent maintenance.

After Harding left, the locomotive caught fire. Firefighters from Nantes shut down the running engine before putting out the fire — which had the unintended effect of stopping power to the air brake system and left the train secured only by the seven hand brakes.

Firefighters and an MMA employee who arrived after the fire left the scene before 1 a.m. The train began rolling downhill sometime after, and at 1:14 a.m. it reached the centre of Lac-Mégantic, where it derailed and exploded.

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